Beacon system



July 4, 1950 H, G. BuslGNlEs 2,513,282

BEACON SYSTEM Filed May 14, 1945 Y 9 Sheets-Sheet 2 ZSTP/CTr/RE 61 15 7FPA/crane 62 Fa/HL ATTORNEY July 4, 1950 H. G. BuslGNlEs BEACON SYSTEM Filed May 14, 1945 9 Sheets-Sheet 3 2 "a P/c run;

TIME /N MIC/90'- 5500/1/05 54.000

INVENTOR. HENRI GI BUS/GN/ES July 4, 1950 H. G. BUslGNlEs BEACON SYSTEM 9 Sheets-Sheet 4 Filed May 14, 1945 E L H m W f sra. 1 .E. w M m o WG C l w ...mvp/0 H w m MM 2 r w 9 f RH@ H ...nwo u VMM 7 w; L s m n E L r A; 1 L NH EOM f n M/wn am /d M1 f v/MW w H6 M M\CCC m 5 1 uw 5 M L mmw my mm 4 l M. MM xn Y 1/ s um ann w N P4 N L A 1% A upm mm Wm m Graw a I I July 4, 1950 H. G. BuslGNlEs BEACON SYSTEM 9 She'ets-Sheet 5 Filed May 14, 1945 NORA/IY July 4, 1950 H. G. BuslGNlEs BEACON SYSTEM 9 Sheets-Sheet 6 Filed may 14, 1945 HP0/7 /40 l 127 Clica/T INVENTOR. HE/v/P/ G. Bus/GMES' July 4, 1950 H. G. BuslGNlEs BEACON SYSTEM 9 Sheets-Sheet 7 Filed May 14, 19.45

maar .m Wamw mm mi M CME n w m Ffa/v 114 A TTG/FIVE' Y July 4, 1950 H. G. BuslGNlr-:s 2,513,282

BEACON SYSTEM Filed May 14l 1945 9 sheets-sheet 8 @df/4FM;

July 4, 1950 H. G. BuslGNlEs 2,513,282

BEAcoN SYSTEM Filed May 14, 1945 9 Sheets-Sheet 9 Patented July 4, QSQ

BEACON SYSTEM Hem-i G. Busignies,-Frest Hills, N. Y., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application'May 14, 1945, Serial No. 593,603

32 Claims. (Cl. 343-10) This invention relates to beacon systems and more particularly to radio beacon systems of the lighthouse type adapted to provide a display of aircraft approaching and in the vicinity of the beacon.

Radio beacon or position indicating systems wherein a display of objects which will serve to reradiate energy may be made on a distantly located receiver such as used on a moving aircraft have been proposed. In order to obtain the displayindications, it is necessary to solve a series of triangles for the various reradiator positions so that the parameters for the display may be properly dimensioned. In such arrangements, in order that the position of the display receiver relative to the transmitter or radio beacon may be indicated both as to spacing and direction, it has generally been required that directive transmission be used at the radio lighthouse together with directive reception on the aircraft; or that at least two radiating sources must be provided at the beacon in known spaced reiationship. These radiators may be either sharply directive or omnidirectional in this latter case. When a directive beacon is provided together with a spaced known reradiating or other radiating source, then directivity need not be used on the aircraft or distant receiver. If omnidirectional radio beacon transmitters in spaced relation are provided, then directive reception must be provided on the aircraft or other indicating receiver in order to secure the proper angular relationship on the display unit as well as to triangulate for determining the distance.

The various types of radio lighthouse systems of this nature are more fully described in my'copending application, entitled Direction Finder System, Serial No. 579,568, led February 24,

In accordance with my present invention, I provide a radio lighthouse type of equipment wherein a position display of reradiating objects may be produced in a receiver spaced from, the lighthouse, in response to energy from a single lighthouse transmitter equipment having a directive radiation characteristic and without directive reception at the receiver.

According to one feature of my invention, I provide what may be termed a three-path radar equipment for producing on the' aircraft or other spaced indicating receiver a position display of the indicating receiver position itself with respect to the radio lighthouse and the relative positions of other reradiating objects provided with suitable repeaters. In order to accomplish this with particular identifying characteristics.

display,`energy from the radio lighthouse is transmitted directionally in successively different directlons. This energy is received at each of a plurality of active repeater stations at least one of which is associated with display equipment.

At the displayrepeater location, a pulseof dif-A ferent characteristics, for example on a diilerent f wavelength, may be repeated. Simultaneously with the repeating of the pulse, control of a position indicator sweep circuit is eiected. This repeated pulse and similar repeated pulses from the other 'active repeaters serve to activate a further repeater at the radio lighthouse station. These pulsesrepeated from the lighthouse are then transmitted preferably omnidirectionally The repeated pulse corresponding to that from the display station serves, upon receipt at the display station, to determine the distance and position of this station with respect to the radio lighthouse. The determined angle and distance parameters serve to establish the base parameter for the indicator sweep so that the pulses repeated from the radio lighthouse in response to all the active repeater pulses will produce indications of the relative location of the various active repeaters. It will thus be seen that a display of all of the active repeating objects will be provided at this display receiver unit. Preferably, the triggering pulses from the lighthouse are transmitted over a sharply directive beam so that only repeaters in line with the beam will be activated to transmit pulses to be displayed at the display unit. The display unit also sweeps around the indicator face in timed relation with the directional shift of the sharp lighthouse beam. Thus, the displays of reradiating units are produced at diierent angular locations dependent upon their angular position with respect to the radio lighthouse. This display will correspond substantially to an azimuthal radar display of the type which would be produced directly at the radio lighthouse station. In fact such a display may be provided at the lighthouse station also in response to the repeated pulses from the distant repeaters. The display, however, may be produced at each repeater location provided the necessary indicator equipment is supplied. v

In accordance with the features of my invention outlined above, it is an object of my invention to provide a distance measuring indication system at a spaced receiver operative in response to signal energy transmitted from a spaced transmitter.

azsiaas It is a further object of my invention to provide a radio. position display indicator and a system for producing radio display of a pluralityl Having determined the distance between the display receiver equipment and the radio beacon by the so-called three-path radar equipment, a radio lighthouse display may also be produced at the indicator equipment by means of the directive beacon and the various reradiating objects both active and passive, in the eld thereof. For this purpose, reected energy not dependent upon special repeated signals may be received at the indicating station. The triangle defined by the radio lighthouse display indicator andA any one of the other reradiating objects may be solved by the joint cooperation of the directive .transmission beam-and the known distance between the display indicator receiver and the lighthouse as determined by the-radar equipment. The display indicator may then be caused to deflect in accordance with thedetermined position indication resultant based on the spacing and the directivity of the transmitter. Accordingly, at the display indicator there may be produced a duplicate indication substantially similai to the one rst described. However, in this instance, passive repeaters or reecting objects will be shown as well as the active repeaters. InV

this type of display, however, the position of objects within an area vdefined by an ellipse including the radio lighthouse and the display indicator itself will not be capable of proper definion. At the display indicator, circuits may be provided to apply alternately the two indication voltage pulses to the display apparatus. If there is any error in the directional or distance indications of either equipment, the two patterns will not then coincide. The greatest possibility of error occurs in the distance measuring system since it is possible that the distance indicating scale of the three-path radar may be based on pulses repeated from the diierent equipment than the station in question. However, because this will result in distortion .of the two patterns, adjustment may be readily made to make the patterns coincide to correct such error. The fact that error exists in the radio lighthouse, indications may be readily ascer.

tained by comparison of the location of fixed known objects on a map with indications of these produced in the omni-display unit itself.

without the necessity of providing directivity at both the transmitter and the receiver or providing a plurality of known reference radiators.

It is a still further object of my invention to provide a receiver display indicator for alter- .nately producing radio lighthouse position displays anda three-path radar type oi position display.

It is a still further object of my invention to providev transmitter and receiver equipment capable of providing a substantially error-free indication of the position of a plurality of radiating objects at one of these -reradiating object points.

While I have "outlined above the general features and objects of my invention, a better understanding. of these objects and features may be had lfrom the particular description of an embodiment thereof made with reference to the accompanying drawings, in which:

Figs. 1A, 1B, 1C constitute a diagrammatic illustration of ,a radio lighthouse system and time chart cycle of operation in accordance with my invention showing several positions of,transmis sion and the' various cycles of operation;

Fig. '2 is a diagrammatic illustration showing the relative position' of a radio beacon lighthouse in accordance with my invention together with a display repeater unit and several repeating objects; 4

3 is a block circuit` diagram of a radio beacon transmitter system in accordance with my invention; I

Fig. 4 is a block circuit diagram illustrating a repeating display receiver in accordance with my invention;

Fig. 5 is a circuit diagram illustrating the notch follow-up circuit shown as a part of the diagram of Figs;

Fig. 6 is a set of curves used in explaining the operation of the circuit of Fig. 5;

Fig. 7 is a circuit diagram of a cubic-law sweep-V curving circuit for use in the system of Fig. 4; Fig. 8 is a pulse width selector which may be used in the circuits of Figs. 3 and 4;

Fig. 9 is a graphical representation serving to explain'the operation of the system of Fig. 8; and Figs. 10 and l1 are illustrative diagrams of the oscilloscope patterns traced by the radio lighthouse and three-path radar display circuits respectively.

I n Fig. 1 is illustrated a time chart of operations of a complete system in accordance with my In order to produce the alternate three-path blocking controls are provided in the receiver equipmentat the display station to route the various received signals to the proper circuit for producing the separate displays.

In accordance with these further features of my invention it is a still further object of my invention to provide a system for producing a display of a plurality of reradiating objects in accordance with the radio lighthouse principle invention illustrating the beacon and display receiver operation sub-cycles for certain of sixty seven time positions I', 2, El, 62 and 66, 6l of a Y rotatable lighthouse beacon,l the other periods between l and 6l and further periods of operation being omitted.V The odd numbered time sequence indications l, 6j and E1 illustrate a part of the sub-cycle of operation relating to the radio lighthouse vdisplay transmission while the even numbered diagrams 2, 62 and 66 illustrate the subcycles oi operation corresponding to the threepath radar display.v

For the radio lighthouse display operation the radio lighthouse 68 transmits a beam of energy sharply directively as shown at 69 and simultalneously transmits omnidirectionally another series of pulses 'I0 preferably at a diierent radio frequency from those transmitted' at 69. The transmission 10 is for sending synchronizing pulses for starting the sweep circuit for -the radio display indicator which may be located, for esample, on an aircraft indicated at 1l.

An ob- Y radial distance up to 50 miles.

8 structing mountain, for example, is illustrated at 12 and another smaller hill 13. A plurality of other aircraft 1l, 15, 16, 11, 18 and 19 are shown in the vicinity of the radio lighthouse.

For the three-path radar display cycle directive beam 69 is modulated with a particular selective or interrogating signal such, for example, as pulses of a given width different from the pulses which may be transmitted in position for the radio lighthouse display. At the same time, the omnidirectional radiations at the different wavelengths may carry a special signal modulation as indicated by lines 80 for the purpose of indicating that beam 69 is passing through the true North. This will be displayed only for the small angle, for example 1, while the beam is in this direction. For the remaining cycles of the threepath radar display transmission, other signals 8l as shown in columns 52 and 66 of the time chart will be transmitted for the purpose of starting the sweep circuit for the three-path radar dis- Play- In explaining the principles of operation of the system, it will be most convenient separately to consider the principles of operation of the two functions which are more or less separately performed, ige. the three-path radar (SPR) function and the radio lighthouse (RLS) function.

The theory of the operation can best be explained by referring to the time chart shown in Fig. 1B. Since this time chart illustrates the combinedoperation of the complete system, including both the RLS and SPR functions which are performed alternately in very rapid succession, it is necessary to disregard the odd numbered columns in the present discussion. The pictures and time graph shown in column 62 of this figure may be taken as best illustrating the principles of operation now being considered.

The upper picture in this column 62 shows the lighthouse B8 emitting a characteristic signal, such as a pulse of width WI which is radiated in a narrow beam G9 at a given frequency, for example, at microwave frequencies. At the same time, this lighthouse is emitting in an omnidirectional manner, pulses 8l of a different, preferably lower frequency, for synchronizing purposes. It will be noted that the narrow beam of microwave radiations passes through the observers own airplane 1| and two other airplanes 14 and 15 (which are all iiying exactly 15 east of north) and nally strikes mountain 12; but this beam just fails to touch a fourth airplane 16 which is located slightly out of line with the others.

The time-graph at the bottom of column 52 represents only the signals existing along one arbitrarily chosen radius, drawn from the lighthouse at an angle 15 degrees east of north. With respect to this narrow region of space, the timegraph shows all signals which may exist at any In this timegraph, the vertical coordinate represents radial distance along the selected l degree azimuth and the horizontal coordinate represents time in microseconds.

Thus, the lower frequency synchronizing pulses 8|' travelling outward from the lighthouse with the speed of light, is shown as a sloping dashed line 82 on the time-graph. This pulse is `assumed to be radiated at a time after the beginning of the complete rotational cycle and, corresponding to zero miles. The maximum range of the equipment, andthe rotational speed of the beacon determine the length of the radiating line.

asiaas At progressive later instants of time, this pulse is shown at progressively greater radial distances, thus forming an oblique line which nally reaches a 5() mile distance about 270 microseconds later. Since the microwave beam shown in the picture of column 62 happens to lie in the 15 degree azimuth represented by the time-graph, the corresponding transmitted microwave pulse 83 is also shown as a straight heavy, solid line, starting at zero distance at the time denoted by 50,833 microseconds and travelling outward in the same wasT as the lower frequency synchronizing pulse just discussed.

Referring now to the second lpicture in column 62, it will be seen that each of the three airplanes 1l, 14 and 15 which was struck by the microwave beam responds by reradiating a diierent'lower frequency response pulse 84 in all directions.A The mountain 12 is shown as merely reecting the same energy at 85 but this reflection of radiations is of no significance at this time since the SPR display is not intended to show passive reections.

In the time-graph, it will be seen that at the three instants when the outward travelling microwave energy passes through the radial distances occupied by the three airplanes 1l, 14 and 15 which are assumed to be at 9 miles, 20 miles and 35 miles, respectively, three lower frequency reradiations are originated at the corresponding distances, and start to travel back toward the lighthouse 68 with a slope corresponding again to the velocity of propagation as shown at 86, 81 and 88.

The second picture in column 62 represents the lighthouse 68 as again sending out a set of microwave pulses 89, which are assumed to be triggered by the arrival of the three lower frequency responses from the three planes. These pulses, however, are not transmitted in beam fashion, but are sent omni-directionally and hence produce very much weaker signal strengths than the original beamed radiations. These pulses are also characterized by a different characteristic, such as a slightly greater width than the original ones sent' on beam 69. The timegraph at the bottom of column 62 clearly shows at 90, 9| and 92 that these special pulses are individually emitted at the instants of reception of the airplane responses. It will be seen for example that at the instant when the response from the nearest plane 1I reaches the lighthouse 68, a special microwave pulse 90 is emitted and commences to travel outward again.

Summing up the above, it appears that for any one airplane, a complete cycle ofgoperation involves three successive transmissions.'v The first of these travels outward on the narrow beam from the lighthouse to the airplane in question; the second takes place omni-directionally and hence some of its energy returns from the plane to the lighthouse; the third transmission travels outward omni-directionally from the lighthouse After a suitable interval, the airplane receives from the lighthouse a weak signal 90 which represents its own response relayed back to itself. Still later this airplane receives two other similar weak signals 9|, 92 from the lighthouse. which represent the responses of the two other airplanes relayed back again to the observers airplane. In between these ,weak signals from the lighthouse, the airplane will also receive two response signals 81, 88 from the airplanes 14, 15 but the arrival of this energy will be of no consequence since the airplane is not conditioned to receive any signals at this wavelength during the cycles when the SPR principle is applied.

The time intervals from the arrival of the synchronizing signal 82 to the arrival of the three special signals 90, 9| and 92 will be seen to be directly proportional to the radial distances from the lighthouse 68 to the three airplanes 1|, 14, which lie in the 15 degree azimuth represented in the time chart. Thus, if a simple radial sweep is applied to an oscilloscope beam on the observers plane, andif this beam is briefly brightened in the usual way, at the instant of arrival of each of these special signals, the distances of all the three planes which are located in the l5 degree direction will be properly displayed.

In order to make this display correct as to azimuth, it is merely necessary to provide for rotating the deiiection System of the oscilloscope in synchronism with the rotation of the lighthouse beam. This may be easily achieved in many ways e. g. by a motor which runs very slightly faster than the beam and a start-stop clutch which releases a deflection system control shaft for rotation in response to a special signal sent from the lighthouse each time its beam sweeps through north.

The above discussion has considered only the special case illustrated in column 62 of Fig. 1B where the beam is in line with the observers airplane 1|. For other cases, however, the operation is essentially the same except that the interrogating signal G9 will not be heard by the observers plane when it is aimed in some other direction. Column 66 of Fig. 1C, shows a case where the rotating beam 69 no longer strikes the observers own airplane 1l nor either of the two o'.hr airplanes 14 and 15 in the sarne 15 degree azimuth, but does strike an airplane 16 at about 16 degrees azimuth, as shown in the top picture of this column. The second picture of this column shows the 16 degree airplane responding wth an omnidirectionally radiated UHF pulse 84, and the lighthouse 68 emitting a special mlcrowave signal 93 in all directions, this emission being triggered by the arrival of the responsive puise from the airplane 1 6.

Referring now to the time-graph at the bottom of column 56, it will be seen that this is quite similar to what would have been shown in column G2 if there had been only one airplane in the l5 degree azimuth so that only one response pulse was returned to the lighthouse and one special radiation was emitted from this lighthouse. Aside from such diierences caused by the presence of only one airplane instead of three, the time-graph of column 66 is further distinguished by the fact that the original powerful microwave pulse 83 from the lighthouse is not shown travelling outward simultaneously with the synchronizing pulse 82 which starts the cycle. Thereason for this is that the timegraph represents only the signals along the arbitrarily se- 8 lected l5 degree radius. while at the instant represented in column 66, the microwave beam is directed slightly to the right of this radius.

The signals received by the observers airplane 1| are all represented in the timegraph of column 66. First the synchronizing pulse 82 is received unaccompanied by interrogating pulse 83; next, the response pulse 84 from the airplane 16 strikes the observers airplane, but this is of no consequence as previously explained. Next, the special pulse 93 omnidirectionally emitted by the lighthouse 68 at the instant of arrival of the response pulse 84 is received by the observers airplane.l Finally the energy 85 reflected by the mountain 12 strikes the observers plane, but without any signicant effect.

As in the case previously considered, the time interval from the arrival of the synchronizing pulse to the arrival of the special pulse is proportional to the radial distance of the airplane being scanned, (i. e., of the airplane 16 in the present case). Thus, the radial sweep of the oscilloscope of the observers plane correctly displays this airplane at the proper radial distance. Since the oscilloscopes deilection system is assumed to be rotating in synchronism with the lighthouse beam, the spot representing this airplane will also be shown at the correct azimuth.

In similar manner, all other airplanes within ell'ective range of the lighthouse will be successively scanned and properly displayed on the oscilloscope of the observers airplane.

In order to avoid confusion in the above discussion of how a SPR. display is produced on board an airplane, consideration has been limited to one airplane 1|. Other airplanes have been considered only as having responding equipments but the receiving and display equipments of these other airplanes have been disregarded. Nevertheless, it should be clear that every single one of the airplanes liying within elective range of the lighthouse may be provided with a full and accurate display similar to that provided on the particular airplane selected for consideration. Each display will show the positions of all airplanes including also the position of the airplane on which the display is given.

Points on the ground which are marked by active repeaters (responder beacons) may also be displayed in the course of the 3PR operation in the same way as above described for the display of airplanes. Natural obstacles and passive repeaters, however, will not be shown on the 3PR display, since any representation of these would give no more information than could be obtained from a printed map.

Briey, the principle of the radio lighthouse system (RLS) type of operation is the same as the principle of operation of conventional radars except that the transmitter is widely separated from the receiver and therefore, parallex correeting means are required to eliminate the distortions resulting from such separation or olfsetting of these two portions or the system, and calculation or determination of the distance must be made.

In the RLS operation of the present invention, the same general principles are used. In this case, however, the transmitter and its sharply beamed slowly rotating antenna are located on the ground, while the receiver with its omni-directional receiving antenna is located on an airplane which may be several miles away.

'I'here is no great diii'iculty in determining the direction of the obstacles whose reflections are being received at any given time. Referring to Fig. 2, if the transmitter 68 is pointing southsouthwest at the moment under consideration, it is clear that all the obstacles "illuminated by such transmitter must be in a straight line extending south-southwest from the transmitting point. Therefore, the deflection coil of the indicating oscilloscope in the airplane can be turned so that at this instant it will deilect the oscilloscope beam radially in the direction representing south-southwest. Since the deecting coil of the oscilloscope is located in the airplane and the rotating beam is on the ground, some synchronizing means is necessary to orient this coil in the same direction as the beam, but such synchronizing means are comparatively simple and reliable.

In order to explain how the correct distance can be determined in spite of the offset between the transmitter and the receiver, reference may be made to Fig. 2. The point L represents the rotary lighthouse or transmitter station on the ground 68, the points O and O represent reflecting objects or other reradiating objects, while the point A represents the airplane 1I which carries the receiving equipment for providing the RLS display now under consideration. The jagged line extending south-southwest from 68 represents the narrow beamed radiation from the lighthouse to the object O and the length of this path from L to O (or O') is denoted by M (or by M') The jagged line from 0 to A represQits the reected energy travelling from the object to the airplane and the length of this line from O (or from to A is denoted by P (or by P'). The airplane A is assumed to be 9 miles southwest of the lighthouse L, and the solid line C represents this distance, i. e., the offset distance between the two parts of the radar.

Consider now one particular pulse of energy radiated south-southwest from the lighthouse toward objects O and O' and reilected from these objects to the airpline 1|. It is clear that the pulse will rst arrive at object O and then will later reach the other object O'. The total time required for the pulse to travel from L to O and then to A, will be proportional to the sum of the distances M-l-P; and in the san-e way the total time between the radiation of the pulse from L and the arrival at A of the pulse reilected from O' will be proportional to M"{-P'. It is, therefore, clear that the pulses from the two objects will not arrive at the airplane 'Il at the same time, but will arrive sequentially. It is also clear that if the airplane 1l is anywhere eYcept directly on the line LO extended (i.'e., for example at a point such as B), the pulse reflected from O will always arrive first and the pulse from O' second, just as in a normal radar. The only difference is that the lengths of the delays are not exactly proportional to the distances from L to' the objects and, therefore, if a linear sweep were used on the oscilloscope, the distances would be distorted. To overcome this, it is necessary to make the sweep circuit non-linear so that it starts moving from the center of the screen very rapidly and then travels slower and slower in accordance with a certain cubic law.

At the particular moment illustrated in Fig. 2, tte beam from L to the two objects is assumed to be aimed south-southwest so that the angle n is 1571/2 degr es. The plane is shown 351/2 degress south of West from the lighthouse so that the angle n' is 125% degrees. Thus, the angle p 10 (which is the difference between n and n') is 32 degrees. The distance C between the airplane and the lighthouse is assumed to be 9 miles.

For these partcula values of angle p and distance C, the length 'of the indirect path M-l-P is about 11.8 miles (assuming that 0 is 7 miles from L) and, therefore, the pulse travelling from the lighthouse to O and thence to the airplane 1| will have to travel 11.8 miles. For synchronizing purposes, another pulse is simultaneously sent directly from the lighthouse to the airplane along path C. Since this direct pulse travels only 9 miles while the indirect reected pulse travels 11.8 miles, the diierence in the path lengths of these two pulses will be about 2.8 miles. Taking the velocity of propagation of all the pulses as .186 mile per microsecond, the airplane will observe a delay of about 15 microseconds between the arrival of the direct pulse from the lighthouse, and the arrival of the indirect pulse reected from object O.

In order to correctly represent the fact that object O is '7 miles from the lighthouse, the sweep voltage which deflects the beam of the oscilloscope in the airplane should, therefore, have such speed that in 15 microseconds it deects the beam to a distance corresponding to 7 miles (i. e., "/3 inches if the desired scale is 3 inches per mile).

For another object such as O', however, (whose distance M is assumed to be l0 miles) the sum of the paths M-j-P will be equal to about 15.3 miles or 6.3 miles longer than the path of the direct pulse. Thus, the delay time for the pulses reilected from O' will be 34 microseconds, or more than twice as great as the delay time for the pulses reflected from O. For correct indication of object O the sweep circuit must, therefore, produce a deilection corresponding to l0 miles (i. e., lo/3 inch deection) in a time of 34 microseconds.

Comparing this latter requirement with the previous requirement, it is seen that in the first 15 microseconds, the beam must move 1/3 inches while in a total of 34 microseconds, it must produce a deflection of only 1/3 inches. Thus, it must travel more than two inches during the first l5 microseconds and only one inch during the next 19 microseconds` If it is assumed that l0 volts must be applied to the oscilloscope for producing 1/3 inch deilection (i. e., for representing one mile of distance), the sweep voltage required must rise from zero to a value of 70 volts in the ilrst 15 microseconds and must then rise more slowly from 70 to 100 volts in the next 19 microseconds.

It is clear that as the beam of the lighthouse rotates farther around so as to increase the angle p to some value greater than the 32 degrees heretofore assumed, the same kind of action above described will take place with respect to the new series of objects which are now in line with the beam. Similarly, for all other values of the angle p. a correspondingly different curve of the sweep circuit is required.

Not only do these curves vary as the angle p changes, but they also vary for diierent values of the distance C. Thus, if the distance C from the lighthouse to the airplane is assumed to be three miles instead of 9 miles, a different family of curves will apply.

Although the curves vary in a seemingly complicated manner with variations of p and also change inscale with variations in C it turns out that they can practically be produced by addingv together two very simple curves as more fully explained hereafter.

The method of producing the RIS display thus boils down to two steps:

(l) Rotating the deflection coil of the oscilloscope in synchronism with the rotation of the lighthouse beam on the ground by means of any simple synchronizing arrangement.

(2) Producing a non-linear sweep which travels rapidly at rst and then more slowly.

The shapes of these curves and, therefore, the speed of travel of the sweeps must be varied for different values of the angle p and the distance C. Thus, in order to obtain a correct indication, it is necessary for the airplane to know its own distance from the lighthouse, as well as its own relative azimuth angle from the lighthouse (measured with respect to the direction of the lighthouse beam at that moment). This relative azimuth angle p is readily found, if the beam is rotating uniformly, by observing the instants when the beam sweeps past the airplane itself and synchronizing a shaft therewith. 'I'he distance C is determined by another mechanism of more or less conventional form hereafter described.

The need for actually knowing the airplanes own position with respect to the lighthouse in order to get a correct RLS indication may at first appear 'as a disadvantage. Actually, however, this is one of the extremely important advantages of the invention, since this makes it possible to check the accuracy of the airplanes own position indication in fool-proof manner, merely by noting whether the various ilxed objects shown on the oscilloscope screen correspond in shape and relative position to the same objects printed on a map. If any error occurs in the self-position finding equipment which determines the airplanes own radial distance or relative azimuth with respect to the lighthouse. a corresponding distortion of the RLS indications will result, so that the natural obstacles and active and passive repeaters will no longer form a picture corresponding to that printed on a map of the terrain. In fact` no conceivable error or series of errors occurring in the mechanism could conceivably result in displaying a correctly shaped indication of the terrain if these determinations of the self-position of the airplane were incorrect.

Although the basic two functions performed bv the proposed system consists of the 3PR and the RLS functions, such as performed by a three path radar and a rotary lighthouse system respectively, it has already been pointed out that for properly producing the RLS display, the airplane equipment requires a knowledge of the airplanes own distance and azimuth with respect to the lighthouse. The determination of these two factors may be made in a great many different ways but the preferred manner of accomplishing this is as follows:

The airplanes own azimuth is determined by noting the time elapsed between the instant when the lighthouse transmits a special signal 80 signifying that its beam is then passing through north of some other fixed reference direction and the somewhat later time when the rotating lighthouse beam sweeps past the airplane. The airplanes distance is determined bv a simple notch followup mechanism or selfadjusting double-gate device which acts, in well known manner, to constantly align itself with a previously selected pulse which is characterjunction with the 3PR operation 0f the systemY since the time-distance relationship is linear for these pulses. It is well known that notch followup devices do not operate as reliably when fed with a very large number of pulses. Accordingly, the only pulses which should be delivered to the notch followup device ,are the special microwave pulses emitted received from the lighthouse at those instants when the lighthouse beam is aligned with the observers own airplane. Referring to Fig. 1B, this means that only the special microwave pulses 89 illustrated in column 62 will be applied to the notch followup mechanism. All other types of pulses, and all the similar special microwave pulses emitted during the other portions of the cycle are screened out before application to the notch followup unit.

Ordinarily, the result will be that only the pulses representing the position of the observers own airplane will be delivered to the notch followup unit, since there will not usually be more than two airplanes lying within plus or minus half a degree from the exact azimuth angle of the observers own airplane, unless there are more than 360 airplanes surrounding the field at one time. In order to illustrate the most disadvantageous conditions, however, the chart of Figure 1 has been drawn on the assumption that three different airplanes 1I, N and l5 are simultaneously ying at the same azimuth angle. Under these conditions, three separate pulses will be applied to the notch circuit in each pulse cycle such as shown in column 62. Even under such conditions, the notch followup mechanism will almost always correctly follow the pulse upbn which it is already set. Thus, if the airplane is the only one ying at its particular azimuth angle at the moment when it enters the eiective field of the lighthouse, its notch which then receives only the pulses corresponding to its own position, will correctly adjust itself to such pulses and will thereafter follow these even during intervals when several other planes are occupying the same azimuth,

Any notch followup device is theoretically subject to the possibility of shifting its tracking so as to follow an undesired airplane if such airplane happens to fly exactly above or below the intended airplane so as to coincide simultaneously in both azimuth and distance. For the sake of economy and simplicity, moreover, it is contemplated to employ a comparatively simple form of notch followup device in the proposed system and, therefore, it is expected that this notch followup device will be subject to the above described change of tracking whenever some other airplane iiies within approximately one-half degree of the exact azimuth of the observers own plane and simultaneously within a certain critical distance zone extending from the observer's plane to 480 yards further out.

More important than the frequency of occurrence of mistracking is the question of its seriousness. In the system of the present invention, the occurrence of a mistracking of the notch V`followup device will constitute only an annoyance but not a hazard. In the rst place, the

76 pilot will see the representation of'his airplane coming gradually closer 'to one of the other spots on the screen until they merge. At this time, no error has yet arisen.- When the merged spot again divides so as to appear as two spots which gradually diverge, the pilot will befully aware Y that there is a possibility of a wrong indication.

If the divergence of the two spots occurs in such a Wag.7 as to result in diierent azimuths, the error, if any, will be immediately corrected. If the planes separate only in respect to their racial distances, and-'if the notch follows the wrong one of the two airplanes, this will be immediately shown by a progressively increasing distortion of this RIS display. The reason for this is that the RIS display depends for its correct shape on the correct position of the notch followup device as previously mentioned.

Whenever an incorrect tracking of the' notch followup mechanism isv thus observed, the pilot can manually return the notch to itsV proper Atracking by adjusting the system until the map assumes a correct form which can readily be observed by comparing it with the undistorted SPR display shown on the same screen.

It is thus clear that the question of possible mistracking of the notch is primarily one of convenience and not., a Yquestion o f l basic misinformation. In fact the system ymay be operated without any notch device at all, Aby arranging for the pilot manually to set the rdistance factor into the system each time he desired to read the RLS display.

To summarize the operation of 'the subcycle for the radio lighthouse system, energy is transmitted in the form of a sharp beam 69 from the transmitter 68. This transmitted energy then may be reiected from the various reflecting objects for reception-on the various craft. Simultaneously with the transmission of the energy in beam 69, energy is transmitted in a plurality of pulses as shown at l0 for the purpose of initiating the sweep circuit on the separate indicating receivers. This pulse operates to produce a linear sweep for the indicator. Pulses 'I0 also are repeated ,by each of the craft carrying the repeaters and these repeated pulses are received on other craft to produce indications of the position of these craft. Thus, on the indicating receiver, for example on cra-ft 1|, there will be received the synchronizing pulses 'i0 starting the sweep circuit and the reflected energy at a wavelength of beam G9 as well as the reradiated energy from other craft. These reected and otherwise reradiated pulses will be timed in accordance with the space position of these objects relative to the sweep circuit so that their position on an, indicator will be clearly set forth.

For a. more detailed understanding of` the operation of the system, the actions of the major units of the transmitting equipment at th'e lighthouse, and of the receiving equipment in the observers airplane will be described in detail in connection with Figs. 3 and 4 which are functional diagrams of transmitting and receiving equipments, respectively, such as may be used at stations 68 and 1|. Since a complete cycle of rotation involves 1200 individual sub-cycles, each of which is initiated by the sending of one synchronizing pulse but may involve three successive complete transmissions and receptions, it is clear that only a small portion of a. complete rotation cycle can be considered.

The'timing chart of Figs. 1A, 1B, 1C shows suflicient sub-cycles to illustrate the action of the special reference or north signals which are sent out at the time the beam is i sweeping I through north, as well as to .illustrate the action of the followup: circuit in the observers airplane which is energized only when the vbeam is sweeping through the 15 azimuth angle occupied by this airplane. In order to cover all these features of interest, these charts have been drawn to show sub-cycles I, 2 which take place while the beam is passing through north, and sub-cycles 6|, 62 which take place when the bea/m is sweeping through the 15 azimuth direction, as well as sub-cycles 66, 6l to cover a portion of'the period'after the beam has left the observers airplane and is scanning another airplane in a, slightly different azimuth.

A complete sequential tracing of all these subcycles would be unnecessarily lengthy and therefore. in the following description only cycles 66 and 81 will be traced in detail since these represent a typical general case where the beam is directed neither north nor toward the observers own airplane (see Fig. 1C) but is aligned with one other airplane and one natural obstacle. After a completev tracing of these two cycles 6B and 61, the special features of other cycles will be briefly noted.

The cycle control circuit 94 of the transmitter station, Fig. 3, delivers control signals selectively over leads 95.-"10 to control the operation of various parts of the circuit. Control circuit 94 may be some form of cyclic switching circuit preferably under control of motor |02 which servesalso to rotate beam E9. At the start of each typical SPR sub-cycle except in the north position, control signals are delivered over leads 95, 96 and |0I. The sub-cycle 66" has been chosen for specific consideration since it best illustrates the general principles. The control signals over lead 95 from circuit 94 triggers the normal pulse width modulator |03 oftransmitter |04 thus causing this transmitter to send out a, high power microwave pulse of normal width W (e. g. 11/'microseconds). The-control signal over lead 96 conditions the electronic switch |05 for routing these pulsesfto the beam radiator |06 and a powerful microwave pulse is radiated in a narrow beam centered on an azimuth angle of 15% degrees. It is assumed, for purposes of description, that this beam is just l 'wide so as to just miss the observers own airplane 1| (Fig. 1C) and the other two airplanes 'I4 and 'I5 in the 15 azimuth. It is assumed, however, that this beam just strikes another airplane 16 which is flying at about a. 16 azimuth as illustrated in Fig. 1C.

At the same time that this microwave beam 69 is emitted from radiator |06 as above traced, the control signal on lead 91 operates pulse generator |01 and causes transmitter |08 to deliver a synchronizing pulse of width W2 of lower frequency UHF carrier which is omnicirectionally radiated by antenna |09, this pulse being shown in line 82 in the time-graph of Fig. 1C. The width of this pulse is chosen to characterize the cycle as a SPR cycle.

In the receiver of Fig. 4 on the observers airplane, the microwave signal (which is assumed to miss this airplane) is not received, but the SPR synchronizing signal of lower frequency is picked up by antenna I I0, Fig. 4, and thence transmitted through coupler to UHF receiver H2. From the output of this receiver 2, this SPR synchronizing pulse passes through width selector ||3 to start linear sweep circuit ||4 and control signal timer H5. Incidentally, this signal also 15 is applied to combining circuit ||6 through coupler 1 and thence to the control grid I8 of oscilloscope |19 so as to cause a bright spot, but this is of no consequence since the beam has not yet started to move away from the center of the screen.

The signal applied te sweep circuiti n4' causes generation of a linear sweep which passes through coupler |20 and combining circuit |2| to, the deiiection coil |22 of the oscilloscope H9..

thus causing the beam to move linearly outward. The signal applied to the control timer causes the latter to deliver a number of blocking and gating signals whichl condition the circuit for 3PR type of operation'as follows: Firstly, the blocking signal applied over line |23 to couplers H1 and |24 prevents this combining circuit H6 from passing during the next 800 microseconds any signals other than .the special width W3 microwave signals characteristic of the three path operation appledover coupler |25. Secondly, the signal applied over line |26 to gate |21 prepares this gate to be opened by a maximum strength microwave signal such as would be produced the beam were pointed at the airplane; since that is not the case during the present cycle, this signal applied to gate |21 is of no consequence. Thirdly, the blocking signal Aapplied from timer ||5 over line |28 to input coupler |29 vol combining circuit |2| blocks the latter in so far as input |28 is concerned so that only the linear sweep waves from sweep circuit ||4 can pass through this combining circuit to the deection coils 22. A

Thus, in response to the UHFsynchronizing signal of width W2, the airplane receiving equipment merely commences a linear outward sweep of the oscilloscope beam and conditions itself to ignore all subsequent signals excepting these special microwave signals reliedupon for the 3PR function.

Referring now to Fig. 1C, it willbe noted that the high `power beamed microwave pulse 83 strikes iirst the airplane 16 and next the mountain 12. The reradiations 85 which take place from the mountain as shown in the second picture of column 66, and in the time graph at the bottom. of this column are of no effect because the receiving equipment is now in condition to display only the special width microwave signals 93. Airplane 16, however, will respond to the powerful beamed microwave pulse in the following manner (for the moment, the diagram of Fig. 4 may be considered as representing the' equipment carried on airplane 16) The circuits of the receiving equipment of airplane 16 receive both the powerful microwave signal and the 3PR UHF' synchronizing signal sub- ,stantially simultaneously: The SPR synchronizing signal produces all the same eiects above traced in connection with the observers plane '.'L The powerful direct beam radiation from the lighthouse is picked up by antenna |30 and is received by receiver |3| from which it passes through the normal width selector |3|A over line |32 to trigger the UHF transmitter |33 thus producing a UHF response as indicated in the second picture in column 66 of Fig. 1C and at 82 on the corresponding time graph of this figure. It should be noted that the transmitter |33 requires a large voltage for triggering and, therefore, cannot be triggered by any but the direct pulse from the lighthouse which `will be several thousand times higher in 'energy than the corresponding reflected pulses. The signal from 'n microwave receiver Isl wm also pas thiouah width, e. g. 2 microseconds. through the electronic switch |05, now in normal the maximum signal selector circuit |30v to perform certain functions, but Vthese will` not be considered at this time since they do not have any relationto the response sent out by the airplane. The corresponding action of the ohserver's airplane 1| will later be described in connection; Y

with cycle 62.

Referring nowto Fig. 1C, it 'will be seen that the response oi` the airplane ,16 retin-ns to the lighthouse 68, and there causes the emission of a special microwave signal 93 of width W3. The corresponding action takes place u follows in the diagram of Fig. 3. The arriving UHF response is picked up by antenna |35 and received by receiver |36 from which it is transmitted not only to a suitable ground display` equipment |31 but also to the special width W3 pulse modulator '|38 of transmitter |04. As a result, this 11ansmitter |04 sends out a microwave pulse of special This pulse passes condition, to'the circular pattern radiator |39 so as to travel outward in all directions as shown in Fig. 1C.

In the receiving equipment of the observers airplane, this special microwave pulse is picked up by antenna |30, received in receiver |3| and delivered through vthe special width selector |40 and coupler |25-to thecombiningcircuit H5.V

Although this circuit ||5 is blocked in respect to its other inputs ||1, |24, it isnot blocked in respect to its input |25 and therefore forwards the signal to the intensity controlling grid ||8 of .oscilloscope ||9. Accordingly, a bright spot is produced on the screen'of this oscilloscope to'representthe position of the airplane 1S, as reported by the lighthouse vv68'.V

Since the deflection coil |22 -oi! this oscilloscope was energized by a linear sweep from I il at the instant of arglval of the SPR synchronizing pulse, the amount' of" radial deection of this beam will, at this instant, correspond tothe time delay between the arrival of such 3PR synchronizing pulse and the arrival of the special microwave signal. This time delay will be proportionate to the radial distance of the airplane 16 fromA the lighthouse as can be seen from the time graph of Fig. 1C; and therefore the spot now produced on the oscilloscope of the observers airplane 12 will be correct with respect to the amount of radial deflection. With respect to the azimuthal correctness of this spot, the rotation of coil |22 is made substantially in synchronism with the rotation of the lighthouse beam so as to show this spot in the correct angular direction. The manner of such synchronism will be described later.A

The SPR cycle is now essentially completed. After the end of an 800 microsecond interval, the timer ||5 will remove the several blocking and gating signals which have temporarily condi.- tioned the equipment for this form of operation, and the receiver willbe ready to commence a new cycle.

The cycle control circuit S4, Fig. 3, delivers con- Y and 95 cause'the emission of a powerful beamedv microwave pulse from radiator |05. The control signal over lead 99 also causes the simultaneous radiation in all directions from antenna |09 gf a UHF synchronizing pulse, but in this case it is the RLS width modulator |4| of transmitter |08 which performs the triggering and therefore the pulse is of such width W4 as to signify the commencement of an RLS type of cycle. The control signal applied to lead serves to block UHF receiver |36 for 800 microseconds, so as to prevent the emission from the lighthouse of the special microwave pulses which are required only in the SPR. cycles.

In the receiver of the observers airplane 1 I, the RLS synchronizing signal is received by receiver ||2 as in the previous case, but this time passes through width selector |42 instead of ||3 since it has a width characteristic W4 of the RLS cycle. 'I'he output of width selector |42 starts the linear sweep circuit I |4 in the same way as in the prior case but does not energize control timer 5. Ac-

. cordingly, no part of combining circuit ||6 nor of combining circuit |2| is blocked. Also, the

gate |21 is not prepared for possible operation.

When the linear sweep circuit ||4 commences to deliver a saw-tooth voltage to the combining circuit |2|, it simultaneously delivers a similar voltage to the cubic-law sweep-curving circuit |43 and accordingly, the latter commences to deliver a suitable correcting voltage of curved characteristics which will be considered in greater detail later. 'Ihis curved voltageoutput of circuit |43 is of such a form that when it is added to the saw-tooth sweep from circuit ||4, with proper restoration of the D. C. axis, the resultant wave will be suitable for the sweep required in the RLS operation. Accordingly, the output of combining and D. C. restoring circuit |2| is applied to the deection coil |22.

Referring now to Fig. 1C, it will be seen that the beam 69 representing the powerful microwave pulse travels outward so as to strike both the airplane 16 and the mountain 12. The airplane responds as before when struck by this beam, and as before, the mountain reiiects some of the microwave energy striking it. Thus, a reflected microwave pulse and a lower frequency UHF response pulse are transmitted in al1 directions from the airplane and mountain, respectively, so as to be received by all other airplanes in the vicinity.

Since the airplane 1B is closer to the lighthouse 68 than mountain 12, its reradiated pulse will reach the observers airplane 1| earlier than the reflected microwave pulse of the mountain. The reception of these pulses will, therefore, be considered in corresponding order.

When the repeated pulse from airplane 16 arrives at the observers own airplane 1|, it is picked up by antenna ||0 and transmitted through coupler to receiver ||2 from which it passes to input coupler ||1 of combining circuit H6 and thence to the intensity control grid ||8 of the oscilloscope ||9. Since the proper sweep voltage has been applied to the deflection coil |22, the radial deflection of the beam at this instant will be correct for representing the distance of the airplane 16 from the lighthouse 68. The deiiection coil |22 rotating in proper synchronism with the lighthouse beam will assure correct azimuth indication. Thus, a spot shown on the oscilloscope will correctly represent the airplane a6 both in azimuth and radial dictance.

A short time later, the reected microwave energy from the mountain will arrive at the plane 1|. This will be picked up by antenna |30, received in receiver |3| and delivered 18 through width selector |3| to input lcoupler |24 of the combining circuit ||6. Since no part of this combiningcircuit is now blocked, this pulse will pass through to the intensity-control electrode ||8 of oscilloscope H9. As in the case of the repeated pulse from airplane 15, this pulse will also produce an indication which is correct in both distance and azimuth but which will generally be of somewhat lower intensity. By providing a separate volume control in the input couplers of circuit H6, signal representations of natural objects and passive repeaters may be adjusted to any desired brilliance, independent of the brilliance employed for the display of active repeaters and other airplanes by the RLS principle and also independent of the brilliance employed for the SPR display.

In the foregoing description the SPR and RLS operation was traced without explaining in detail all the features. For example, the synchronous rotation of coil |22 was assumed. How this synchronism may be accomplished will now be described.

Referring to Fig. 1A, it will be seen that when the beam of the lighthouse is sweeping through north, the successive cycles of this lighthouse are performed as usual, except that the regular 3PR synchronizing signal ordinarily transmitted at the start of each even sub-cycle is temporarily replaced by a slightly modied signal for characterizing the north orientation of the beam.

This special north synchronizing signal may be of a Width W5 only slightly different from width W2 so it will pass through the width selector ||3 of the receiving equipment, so that the 3PR cycle takes place in the same manner as usual. This special north signal, however, will also pass through width selector |44 so as to energize start-stop clutch |45 over line |40. Width selector |44 is made more selective than selector ||3 so that pulses of width W2 will not be passed.

An accurate speed motor |41 with suitable gear reduction and speed control drives the input shaft |48 of clutch |45 at a speed very slightly faster than the beam rotation which has been chosen for illustration at 50 R. P. M. When the system is first put in operation, the motor will turn the input shaft |48 of clutch |45 but the output shaft |49 will not be able to rotate until this clutch is tripped by an electric impulse. The next time the lighthouse beam swings through north, the clutch will be tripped so as to permit the output shaft to make one rotation. Since the lighthouse beam is assumed to rotate at exactly 50 R. P. M., while the motor turns slightly above this speed, the output shaft of start-stop clutch will complete its cycle a few milliseconds before the beam of the lighthouse again reaches north and will pause for a correspondingly brief interval before it is again released to commence a new cycle. Thus, the output shaft |49 of clutch |45 rotates in substantial synchronism with the lighthouse beam and its angular position at every instant closely corresponds to that of the lighthouse beam.

In the particular arrangement illustrated, the output shaft |49 of clutch |45 is directly connected to magnetic deflection coil |22 so that a fixed map form of display will be produced,

with the north direction on the scope in a. fixed' position with respect to the screen, e. g. always at the top of the screen. This form of indication has the advantage of being consistent with the fixed central representation of the lighthouse,

vengraved lightly on its surface.

19 which results naturally from the simple forms of SPR and RLS displays. In order to show the heading of the plane, as well as its position on such "xed map display, a heading indicator |50 is provided. This may take the form of a transparent disc with a, large number of arrows This heading indicator dial |50 is rotated by a compass repeater controlled by some sort of compass.

It is clear that a self-orienting-map form of display may be given if preferred by providing a differential gear train between clutch |45 and coil |22 and connecting compass |5| to this gear train. Then the indication would orient itself so that the top of the screen would correspond to the heading of the airplane. In such case, the north direction would be shown on the screen by a dial like 150 or by other suitable means.

In order to produce radio lighthouse indica tions it will be recalled that the sweep circuit at the indicator must take into consideration the distance C indicated in Fig. 2. Since the receiver is normally on a moving craft this distance must be continuously determined. In accordance with my invention, when the beam of the lighthouse B8 actually sweeps over the observers airplane 1|, certain additional steps are performed for the .purpose of determining the airplanes self-position. One of these special operations, the determination of self-azimuth, is performed during every sub-cycle. The other special operation, the determination of selfdistance, is performed by the notch follow-up unit |52 which is actuated only during the even sub-cycles. A description of sub-cycle 62 will, therefore, serve to illustrate both these operations.

In general, sub-cycle 52 takes place like any other SPR sub-cycle, described in connection with sub-cycle B6. Because of the fact that the powerful beamed microwave pulses directly from the lighthouse strike the airplane during this cycle, certain additional actions take place.

When such a powerful microwave pulse arrives at the observers plane, it is picked up by antenna |30 and received by receiver |3l. From the output of this receiver, the powerful pulse passes through width selector |3|A to trigger transmitter |33 thus causing the emission ofa response signal, as previously described. Aside from producing this response, however, two other important enects are produced in the observers airplane. One of these eiects provides a determination of self-azimuth and the other provides a determination of self-distance.

For the -purpose of self-azimuth determination, the powerful pulse from receiver |3| is applied to maximum signal selector |34 which is biased to select only the most powerful' of the pulses delivered during one complete rotational cycle. From the output of this selector |34, the pulse is delivered to start-stop clutch |53 over line |54A; This start-stop clutch is similar to clutch |45 previously referred to and is driven by the same motor |41. The output shaft of |54 is, therefore, synchronized in essentially the same manner as the output shaft |48, excepting that the reference point for the synchronization is not the instant when the lighthouse beam passes north, but rather the instant when this beam sweeps over the observers airplane. Thus, the angular position of the output shaft |55 of this clutch |53 constantly corresponds with angle p, Fig. 2. Since the angle p is one of the param- Veters required in the case of RLS operation,

20 the rotation of this shaft is suitable for application to the cubic-law sweep-curving circuit |43.

'The other parameter required by this sweepcurving circuit |43 is the radial distance of the observers airplane from the lighthouse. This parameter also is obtained during the brief period when the lighthouse beam is sweeping past the observers airplane.' Only the 3PR type of cycles occurring during this interval are employed, thus greatly vreducing the number of pulses applied to the notch follow-up |52 so as to improve the operation of the latter.

In order to thus pass only the special microwave pulses emitted from the lighthouse during the instants of alignment of the main sweep of the observers own airplane, gate circuit 21 is provided which opens only when it simultaneously receives signal voltages from control signal timer ||5 and maximum signal selector |34 applied over branch line |56. Since the selector |34 delivers signals only when the strong pulses of the direct beam strike the airplane, while timer ||5 delivers its gating and blocking control signals only during cycles of the SPR type. it will be clear that gate |21 will pass only the maximum pulses occurring during the SPR interval. Extra security is provided by the special Width selector |40 which will pass only the special microwave pulses of width W3.

Referring now to Fig. 1B, it will be seen that the only special microwave signals -92 which are received simultaneously with the direct powerful microwave pulses 83 are the signals shown in column 62 which represent the radial positions of the three airplanes assumed to be located at azimuth angle I5. In the receiver equipment of Figui, therefore, the only pulses which can pass from receiver |3| through the special width selector |3|A and gate |21 are the three successive pulses representing the radio positions of these three airplanes.

To provide the necessary reference pulse for the notch follow-up circuit, the synchronizing pulse from thevoutput of width selector ||3 is delivered to this unit |52 over line |51.

The control signal timer pulses applied to gate circuit |21 over line |26 are timed with the synchronizing pulses of width W2. These pulses are medium length rectangular pulses |58 of Fig. 6. These pulses |58 occur only during the time when the sharp directive radiation is being keyed for the three-path radar operation so that only the pulses occurring during this time interval of the sub-cycle will be present while these keyed pulses are being applied to gate |21. Furthermore, the maximum selective circuit |34 shown in Fig. 4 also will produce a controlling pulse only during the relative narrow interval when the microwave beam is directed toward the indicating receiver. This pulse while relatively short with respect to a complete rotation cycle is quite long with respect to pulses |58 and may be represented at |59 of Fig. 6. These two positive pulses 58 and |59 serve to bias gate circuit |21 to pass the received signal pulses incoming from Width selector |40. As shown in Fig. 6, there are three such pulses |60, ISI and |62. For the purposes of the notch gate system, it is desired that only one of these three pulses be selected to the exclusion of the others.

If it is assumed that pulse |60 is the repeated pulse corresponding to the transmission from the observers plane, this pulse should be selected. In order to secure this selection and to have the device follow it up so 'as to maintain the indication in a position representative of distance, the notch follow-up unit |62 is provided. As shown in Fig. 5, the selected synchronizing pulses of width W2 may be applied over line |51 to a variable delay device |63. I'his variable delay device is driven by a motor |69A which rotates to advance the delay of the variable delay device onehalf the width of the selected pulse |60 for each normal rotation of the radio beacon, when driven in one direction and retard the delay one-half this distance when driven in the other direction. The selected pulses |60 therefore are caused normally to fall between two control pulses applied to notch gate circuits |64, |65.

These gate control pulses are shown in Fig. 6 at |66 and |61. Pulses |66 and |61 may be derived directly from the incoming pulses. 'Ihe output of the variable delay device |63 which may, for example, be a trigger circuit of the multivibrator type, will generally be relatively wide pulses with sloping sides. In order that they may be used properly for control, these pulses must be narrowed down preferably to a width less than the normal separation that is to be maintained between craft using the system. It will be clear, however, that these pulses should be sumciently long in time duration so that the craft will not pass beyond such a pulse in two or three seconds of time. This is desirable since. should the signal fade for two or three revolutions of the lighthouse transmitter, the craft might pass completely beyond the notch gate control unit and so the follow-up could not be properly performed. Accordingly, the pulses from the output of delay device 6 are passed through a Shaper network |68 which serves to reshape these output pulses and narrow them down. These output pulses from |68 are applied to notch gate |64 so as to bias it sumciently positive to pass any pulses applied thereto. Thus, any output pulses from gate circuit |21 which occur during the application of pulse |66 to notch gate |64 will therefore be passed on to the motor reversing control mechanism |69.

Output pulses from delay device |63 are also applied over a fixed delay circuit |10 to a second Shaper circuit |1| and from there to notch gate |65. These pulses correspond to |61 shown in Fig. 6 and are delayed suciently to provide a time gap greater than the width of output pulses from gate circuit |21. Pulses from |21 are also applied to notch gate |65 and, if they are applied during the interval when pulses |61 are present, from there to motor reversing control |69 to cause motor |69A to operate one revolution in the opposite direction to that produced by pulses from notch gate |64. It will therefore be seen that as long as pulse |60 is properly timed with respect to pulses |66 and |61 that the motor |69A will remain stationary and its shaft |12 will correspond in angula position to the distance of the receiver from the radio lighthouse station. However, as the craft carrying the receiver moves, the pulse |60 may be displaced to one side as shown in the right hand side of Fig. 6 causing pulse |60 to overlap pulse |66. 'Ihis increased voltage will be passed through notch gate |64 and applied to the motor reversing control causing motor |69A to operate in one direction for one revolution. This will vdisplace the variable delay device sufficient to move the notch gate pulses |66 and |61 over one-half the width of pulse |66. Accordingly, if the craft does not move the next revolution of the beacon, the pulse |60 will again fall within the notch and the shaft will remain stationary. However, if the craft con- 22 tinues to move in distance tending to displace pulse |60, the gate pulses |66 and |61 will tend to follow it up so as to maintain shaft |12 at all times substantially aligned with the distance indication.

The notch gate pulses |66 and |61 are initially lined up by means of a manual control knob |13. This knob may be controlled in position by observation of the indications produced on the indicator H9, adjustment being made until such time as the indications properly coincide with the position on the map. It will be clear that if desired the entire notch follow-up unit may be eliminated and manual adjustments made for every reading that is to be taken. This, however, entails rather tedious observation of the instrument and does not permit such rapid operation as is desired.

If no fading of the signal need be anticipated the notch gate system may be considerably simplified. With such an arrangement only one notch gate pulsing circuit need be provided instead of the two shown in Fig. 5. Motor |6SA may then be made normally to advance the notch one-half the width of the applied pulses for each rotation of the beacon. Should a pulse fail to appear on the notch gate, the motor may then be caused to rotatebackwards one revolution thus stepping the delay device back one step. This would thus keep shaft |12 hunting a small distance back and forth across the receiving distance. However, the use of a single pulse would also permit the distance measurements to be made more accurately so that less separation between craft in the same azimuth direction could be tolerated.

In the foregoing description of the RIS type of operation, the manner of operation of the sweep curving circuit |43 of Fig. 4 was not described, but it was merely assumed that this circuit produced the necessary correcting voltage under control of shafts representing angle p and distance C respectively, Fig. 2. 'I'he required correcting voltage was. then described as being combined in circuit |2| with the linear saw-tooth sweep voltage from sweep circuit ||4. The combining circuit |2| was also assumed to properly restore the zero axis in known manner. The resulting output in circuit 2| was then assumed to correspond to the required cubic-law sweep-voltage.

In order to consider more specifically the manner of attainment of these sweep voltages, the trigonometric relationships of Fig. 2 will be considered. In the triangle whose sides are C, M and P, and whose opposite angles are c, m and p, respectively, the usual cosine law for determination of one side (in terms of the other two sides and the angle included between them) may be written as follows:

P: C2+M2-2CM cos p if D represents the difference between the paths of the direct pulse from L to A and the indirect pulse from L via O in Fig. 2, it is clear that D=P+M-C; and if .i is the corresponding observed delay between the arrivals of the direct and indirect pulses A=(P+M-C) +11 (where v is the velocity of propagation in miles per microsecond, i. e. approximately .186). From the definitions of D and A, it is clear that 

